1. Field of the Invention
This invention relates to a method and an apparatus for determining the moisture of a running material web, in particular a paper web or paperboard web.
2. Description of the Related Art
The moisture of the running paper web is one of the most important measurement and control variables in the paper production process. For fast control operations, the measured values have to be precisely established online. The measured moisture value can then be used for measurement and control tasks.
Today, moisture is determined in the paper industry usually by optical methods, such as using characteristic wavelengths of water and fibers measured spectroscopically. The percentage of moisture content of the paper is then calculated using the transmission or reflection response. A disadvantage of this method is the high technical outlay, the high price and the large dimensions of the corresponding sensors.
Another technology used in the paper industry for determining moisture is based on an indirect method such as measuring the electrical conductivity of the paper, which stands in correlation to the quantity of water contained in the paper (cf. U.S. Pat. No. 6,099,690). The disadvantages of this known method are that the conductivity depends on the chemical composition of the paper and that the measurement is dependent on the dielectric constant of the paper and on the distance of the sensor element from the paper.
It was already proposed to determine the microwave resonance frequency of a cavity resonator implemented in a sensor as a way of measuring the moisture. The resonance response of such a cavity resonator changes with the moisture of the paper. The disadvantage of this method is, among other things, that the sensor has relatively large dimensions.
Several developments and advances in microwave technology for the communications industry in the frequency range important for moisture measurement, namely 1 to 100 GHz, have resulted in a miniaturization of the corresponding components and a drop in their prices. New avenues for further development have thus opened. Described in EP 1 000 314 B1 is an open cavity sensor for distance measurement.